FORM 2
The Patents Act, 1970
(39 of 1970)
As amended by the Patents (Amendment) Act, 2005
&
The Patents Rules, 2003
As amended by the Patents (Amendment) Rules, 2006
Complete specification
(See section 10 and rule 13)
TITLE OF INVENTION
A current limiting device
APPLICANTS
Name : CROMPTON GREAVES LIMITED
Nationality: Indian Company
Address : CG House, Dr Annie Besant Road, Worli Mumbai 400025, Maharashtra, India
PREAMBLE TO THE DESCRIPTION
The following specification particularly describes the nature of this invention and the manner in which it is to be performed:

FIELD OF INVENTION
This invention relates to a current limiting device.
Current limiting devices are used to switch inductive or capacitive loads in power distribution systems.
BACKGROUND OF INVENTION
A current limiting device generally used in a power distribution system comprises a vacuum interrupter and an auxiliary circuit connected to a power supply and an inductive or capacitive load in the power distribution system. The vacuum interrupter comprises a fixed contact system and a movable contact system located in a vacuum sealed ceramic body. The auxiliary circuit comprises a mechanical switch and a resistor connected in series. During switching operation, there will be initially a high inrush current flow in the system. This inrush current not only reduces the life of the current limiting device but also generates stresses in the entire system. The sequence of operation of the current limiting device is controlled such that initially the auxiliary circuit will be closed to provide a high resistive path for the high inrush current. Following this, the vacuum interrupter will be closed to provide a low resistive path and a continuous flow of current through the low resistive path. The time delay between the operations of the auxiliary circuit and vacuum interrupter is set for a few cycles such that when the current reduces and stabilizes, it will flow through the low resistive path provided by the vacuum interrupter. The mechanical switch has limited life in that it can perform only a few contact making and breaking operations. This calls for periodic replacement of the auxiliary contact thereby increasing the maintenance cost of the auxiliary circuit. Besides, the vacuum
2

interrupter and auxiliary circuit are external to each other and comprise a large number of components because of which the system is bulky and costly.
OBJECTS OF INVENTION
An object of the invention is to provide a current limiting device which comprises
reduced number of components and is compact and cost effective.
Another object of the invention is to provide a current limiting device which has increased life.
DETAILED DESCRIPTION OF INVENTION
According to the invention there is provided a current limiting device for switching an inductive or capacitive load in a power distribution system, the device comprising a vacuum interrupter comprising a fixed contact system and a movable contact system located in a vacuum sealed ceramic body, the vacuum interrupter further comprising an auxiliary contact comprising a contact ring fitted at the upper end of a compression spring disposed within the center shield around the fixed electrode of the fixed contact system in spaced apart relationship with the fixed electrode, the lower end of the compression spring being fixed to the fixed end plate of the fixed contact system and the upper end of the compression spring with the contact ring protruding out of the fixed contact, the contact ring being isolated from the fixed electrode and fixed contact and a wound resistor disposed around the fixed electrode spaced apart from the fixed electrode, the wound resistor being connected to the contact ring and the
3

outer end of the fixed electrode, the movable contact of the movable contact system being broader than the contact ring and adapted to close against the contact ring prior to closing on the fixed contact.
The following is a detailed description of the invention with reference to the accompanying drawings, in which Figs 1, 2 and 3 show the current limiting device of the invention according to an exemplary embodiment thereof in the fully open, auxiliary contact closed and fully closed positions. The device 1 as illustrated in Figs 1 to 3 of the accompanying drawings comprises a vacuum interrupter 2 comprising a fixed contact system 3 and a movable contact system 4 located in a vacuum sealed ceramic body 5. The fixed electrode, fixed contact and center shield of the fixed contact system are marked 6, 7 and 8 respectively. The center shield minimises metal vapour deposition on the ceramic body due to arcing during the switching operation of the vacuum interrupter. The fixed seal cup and end plate are marked 9 and 10, respectively. The movable electrode, movable contact, bellows and bellows shield of the moving contact system are marked 11, 12, 13 and 14 respectively. The bellows shield protects the bellows from metallic vapours generated during switching operations of the vacuum interrupter. The stacking ring, movable seal cup and end plate, guide bush and arresting cap are marked 15, 16, 17, 18 and 19 respectively. The guide bush guides the axial movement of the movable electrode. The outer ends of the movable electrode and fixed electrode are provided with threaded holes 20 and 21 respectively for connecting them to a power supply (not shown) and an inductive or capacitive load (not shown) by fitting flexible conducting cables (not shown) from the power supply and the load in the threaded holes 20 and 21 respectively or vice versa. An auxiliary contact comprising a contact ring 22 is fitted
4

at the upper end of a compression spring 23 disposed within the center shield around the fixed electrode of the fixed contact system in spaced apart relationship with the fixed electrode. The lower end of the compression spring is fixed to the fixed end plate of the fixed contact system. The upper end of the compression spring with the fixed contact protrudes out of the fixed contact. 24 is an auxiliary fixed end plate fitted at the outer end of the fixed electrode. 25a and 25b are auxiliary seal cups fixed to end plates 10 and 24 and to a ceramic cylinder 26 disposed over the fixed electrode in spaced apart relationship therewith. 27 is a wound resistor (cylindrical or spring type) disposed over the ceramic cylinder in spaced apart relationships with it and fixed to the outer end of the fixed electrode and to studs 28a and 28b which in turn are fixed to the fixed end plate. The contact ring is isolated from the fixed electrode and fixed contact by the gap 29 between the fixed electrode and fixed end plate and by the ceramic cylinder. The movable contact 35 is broader than the contact ring and the current limiting device in the fully open position is as shown in Fig.l. During switching of the device, the movable contact moves towards the fixed contact and first closes against the contact ring (Fig 2). The contact ring and wound resistor provide a large surface area and a high resistive path for the inrush current. The arcing between the movable contact and the contact ring will be considerably reduced because of the large surface area and high resistive path offered by the contact ring and wound resistor. The movable contact continues to slide forward under compression of the compression spring until it closes against the fixed contact. The inrush current reduces and stabilises by this time and the movable and fixed contacts of the vacuum interrupter in their closed position ensures a low resistive path for the current to flow to the load (Fig 3).
5

Typically, the fixed and movable electrodes and stacking ring are made of copper, fixed and movable contacts and auxiliary contact ring are made of copper chromium alloy, fixed and movable end plates, auxiliary end plate, bellows, bellows shield, arresting cap, compression spring, studs and center shield are made of stainless steel, fixed and movable side seal cups and auxiliary seal cups are made of copper nickel alloy, ceramic body and ceramic cylinder are made of high alumina and bushing is made of nylon. It is to be understood that the above examples of the materials of the components are only preferences and that the components can be made of other vacuum compatible materials. The auxiliary circuit configuration and construction can vary. Such variations are to be construed and understood to be within the scope of the invention.
According to the invention, the auxiliary circuit comprising mechanical switch has been eliminated and the auxiliary circuit is built into the vacuum interrupter to provide both the high resistive path and low resistive path within the same switching device. The number of components of the switching device is reduced and it is rendered compact and cost effective. The life of the switching device is also increased.
6

We claim
1. A current limiting device for switching an inductive or capacitive load in a power distribution system, the device comprising a vacuum interrupter comprising a fixed contact system and a movable contact system located in a vacuum sealed ceramic body, the vacuum interrupter further comprising an auxiliary contact comprising a contact ring fitted at the upper end of a compression spring disposed within the center shield around the fixed electrode of the fixed contact system in spaced apart relationship with the fixed electrode, the lower end of the compression spring being fixed to the fixed end plate of the fixed contact system and the upper end of the compression spring with the contact ring protruding out of the fixed contact, the contact ring being isolated from the fixed electrode and fixed contact and a wound resistor disposed around the fixed electrode spaced apart from the fixed electrode, the wound resistor being connected to the contact ring and the outer end of the fixed electrode, the movable contact of the movable contact system being broader than the contact ring and adapted to close against the contact ring prior to closing on the fixed contact.
Dated this 4th day of January 2007

7

Abstract
A current limiting device for switching an inductive or capacitive load in a power distribution system. The device (1) comprises a vacuum interrupter (2) comprising a fixed contact system (3) and a movable contact system (4) located in a vacuum sealed ceramic body (5). The vacuum interrupter further comprises an auxiliary contact comprising a contact ring (22) fitted at the upper end of a compression spring (23) disposed within the center shield (8) around the fixed electrode (6) of the fixed contact system in spaced apart relationship with the fixed electrode. The lower end of the compression spring is fixed to the fixed end plate (10) of the fixed contact system and the upper end of the compression spring with the contact ring protrudes out of the fixed contact. The contact ring is isolated from the fixed electrode and fixed contact (7). A wound resistor (27) is disposed around the fixed electrode spaced apart from the fixed electrode. The wound resistor is connected to the contact ring and the outer end of the fixed electrode. The movable contact (12) of the movable contact system is broader than the contact ring and is adapted to close against the contact ring prior to closing on the fixed contact (Fig 1).